xFexO3 Perovskites Research Articles

Experimental and theoretical study on La0.5K0.5Mn1−xFexO3 perovskite catalysts for mild temperature soot combustion and simultaneous removal of soot and NO

In order to control high emissions of soot and NOx from diesel engines, this research utilizes highly efficient perovskite oxide catalysts to simultaneously remove soot and NO. A series of La0.5K0.5Mn1-xFexO3 catalysts were prepared by citric acid complexation method, and the active sites and structure stability of LaMnO3 perovskite catalyst were investigated according to the first principle. Research results show that K+ doping causes the transformation of Mn3+ into Mn4+ in LaMnO3 catalyst system and forms more oxygen vacancies. Meanwhile, Fe3+ doping leads to the octahedron distortion of MnO6 and FeO6 in catalyst system, which enhances the stability of catalyst structure. The oxygen vacancy formed by doping K+ and Fe3+ accelerates the migration rate of lattice oxygen and improves the catalytic activity of perovskite catalyst for soot oxidation. With the increase of Fe3+ doping, the proportion of adsorbed oxygen and high valence active ions in catalyst system increases. Moreover, the alkaline sites on the catalyst surface are favorable for the adsorption of acid gas NO. The La0.5K0.5Mn0.67Fe0.33O3 catalyst has the highest catalytic activity, the ignition temperature of soot oxidation drops to 291 °C, and CO2 selectivity exceeds 99.9%. At 370 °C, the conversion rate of NO reaches the maximum value of 61.7%.

Highly Resistant LaCo1−xFexO3 Perovskites Used in Chlorobenzene Catalytic Combustion

The stability of LaCo1−xFexO3 perovskite structures (x = 0; 0.25; 0.5; 0.75; 1) was studied in the combustion of chlorobenzene. This family of catalysts was synthesized by the citrate method obtaining pure structures. The Fe doping in the original structure induces electronic environments capable of generating the Co2+/Co3+ redox couple. The characteristics observed in bulk are perfectly reflected on the surface, favoring a high resistance of the solids to chlorine poisoning. Superior stability under reaction conditions was observed in the phase with the lowest Fe content (x = 0.25), remaining stable at 100% combustion of chlorobenzene during 100 h, not observing intermediate reaction products. These results open up a new avenue for designing and fabricating high-performance catalysts in the environmental field

Cover Feature: Spray‐Flame‐Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low‐Temperature Heating (ChemElectroChem 12/2020)

The Cover Feature shows a spray-flame reactor used to synthesize perovskite catalyst nanoparticles. The blue methane/oxygen pilot flame at the bottom ignites a combustible precursor aerosol injected via a centric nozzle. Nanoparticle formation downstream of the reaction zone leads to metastable, highly active LaCo1–xFexO3 perovskite catalysts with improved oxygen evolution reaction activity. The image was taken by Samer Suleiman. More information can be found in the Aricle by B. Alkan et al.

Spray‐Flame‐Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low‐Temperature Heating

AbstractSpray‐flame synthesis was used to produce high‐surface‐area perovskite electrocatalysts with high phase purity, minimum surface contamination, and high electrochemical stability. In this study, as‐prepared LaCo1–xFexO3 perovskite nanoparticles (x=0.2, 0.3, and 0.4) were found to contain a high degree of combustion residuals, and mostly consist of both, stoichiometric and oxygen‐deficient perovskite phases. Heating them at moderate temperature (250 °C) in oxygen could remove combustion residuals and increases the content of stoichiometric perovskite while preventing particle growth. A higher surface crystallinity was observed with increasing iron content coming along with a rise in oxygen deficient phases. With heat treatment, OER activity and stability of perovskites improved at 30 and 40 at.% Fe while deteriorating at 20 at.% Fe. This study highlights spray‐flame synthesis as a promising technique to synthesize highly active nanoscale perovskite catalysts with improved OER activity.

Spray‐Flame‐Synthesized LaCo1−xFexO3 Perovskite Nanoparticles as Electrocatalysts for Water and Ethanol Oxidation

AbstractCoupling electrochemical generation of hydrogen with the concomitant formation of an industrially valuable product at the anode instead of oxygen can balance the high costs usually associated with water electrolysis. We report the synthesis of a variety of nanoparticulate LaCo1−xFexO3 perovskite materials through a specifically optimized spray‐flame nanoparticle synthesis method, using different ratios of La, Co, and Fe precursor compounds. Structural characterization of the resulting materials by XRD, TEM, FTIR, and XPS analysis revealed the formation of mainly perovskite‐type materials. The electrocatalytic performance of the formed perovskite‐type materials towards the oxygen evolution reaction and the ethanol oxidation reaction was investigated by using rotating disk electrode voltammetry. An increased Fe content in the precursor mixture leads to a decrease in the electrocatalytic activity of the nanoparticles. The selectivity towards alcohol oxidation in alkaline media was assessed by using the ethanol oxidation reaction as a model reaction. Operando electrochemistry/ATR‐IR spectroscopy results reveal that acetate and acetaldehyde are the final products, depending on the catalyst composition as well as on the applied potential.

Hyperfine properties of La(V1−xFex)O3 compounds

LaV1−xFexO3 perovskites were synthesized in the vanadium-rich concentration range (i.e., x < 0.5) and characterized structurally and for the hyperfine properties of the iron nuclear probe. The aim of this investigation was to better understand the physical transformations that take place in the undoped compound (LaVO3) at low temperatures. For that, X-ray diffraction analysis and, more extensively, 57Fe Mossbauer spectroscopy were applied. The results revealed that the LaV1-xFexO3 vanadium-rich perovskites are orthorhombic at RT, and their lattice parameters decrease with increasing vanadium concentration. Lowering the temperature, the system becomes magnetic, with the iron moment freezing progressively. The presence of two magnetic subspectral components obtained at the lowest measurement temperatures suggests that the vanadium-rich samples, including LaVO3, undergo a phase transition from an orthorhombic to a monoclinic structure at low temperatures.

Catalytic Reduction of NO by CO over LaMn1−xFexO3 and La0.8A0.2Mn0.3Fe0.7O3 (A = Sr, Cs, Ba, Ce) Perovskite Catalysts

In this study, LaMn1−xFexO3 (x = 0, 0.3, 0.5, 0.7, 1) and La0.8A0.2Fe0.7Mn0.3O3 (A = Sr, Cs, Ba, Ce) perovskite oxides were synthesized by sol–gel method and their activities were evaluated in catalytic reduction of NO by CO. Perovskite catalysts were characterized by XRD, BET, H2-TPR, XPS and SEM. Synthesized perovskites present a high activity for the catalytic reduction of NO by CO, LaMn0.3Fe0.7O3 Show the highest activity among LaMn1−xFexO3 perovskite catalysts (71 % CO conversion and 82 % NO conversion at 350 °C). The effect of partial substitution of Sr, Cs, Ba and Ce in A-site was also examined on the structure and catalytic activity of LaMn0.3Fe0.7O3 perovskite catalyst. La0.8Sr0.2Fe0.7Mn0.3O3 and La0.8Ce0.2Fe0.7Mn0.3O3 present the highest activities among La0.8A0.2Fe0.7Mn0.3O3 perovskites. The introduction of Sr2+ and Ce4+ in A-site of perovskite change the reducibility of B-site cations and Fe4+/Fe3+ and Mn4+/Mn3+ ratios, and increase Oads/Olatt ratio and these factors create structural defects in perovskite which lead to higher catalytic activities.

KNO3 supported on three-dimensionally ordered macroporous La0.8Ce0.2Mn1−xFexO3 for soot removal

The KNO3 supported on 3DOM La0.8Ce0.2Mn1−xFexO3 perovskites have been prepared and used for soot removal. The presence of potassium nitrate can accelerate the soot combustion at both low and high temperature. The catalysts exhibit high activities for soot combustion.

Structural, magnetic and magnetocaloric properties of La0.67Ba0.22Sr0.11Mn1 − xFexO3 nanopowders

The influence of Fe-doping on the structural, magnetic and magnetocaloric properties of La0.67Ba0.22Sr0.11Mn1 − xFexO3 perovskite was investigated. These samples were synthesized by Sol–Gel method. X-ray diffraction (XRD) measurements for all samples indicates that all samples, except that with x = 0.2 and 1.0, crystallize in the orthorhombic single-phase with Pnma space group. The Rietveld refinement of the X-ray nanopowder data confirms this result. Room-temperature transmission Mössbauer spectra for all samples except that with x = 1.0 consisted of doublets with similar isomer shift (δ) values consistent with Fe3+ valence state. The spectrum for the sample with x = 1.0 showed the presence of an additional magnetically ordered phase and showed evidence of the presence of Fe4+ valence state. Magnetization measurements versus temperature under magnetic applied field of 0.05 T showed that all of our investigated samples exhibit a paramagnetic–ferromagnetic transition at low temperature except that with x = 1.0. The Curie temperature decreased with increasing Fe content from 345 K for x = 0–80 K for x = 0.3. The magnetic entropy change |ΔSM| for the parent compound was evaluated using Maxwell's relation. The maximum value of the magnetic entropy change obtained from the M(H) plot data is |ΔSMMax| = 2.26 J kg−1 K−1 for applied magnetic field of 5 T. Our result on magnetocaloric properties suggests that the parent sample La0.67Ba0.22Sr0.11MnO3 could be a candidate refrigerant for low temperature magnetic refrigeration.

An in situ electrical conductivity study of LaCoFe perovskite-based catalysts in correlation with the total oxidation of methane

LaCo1−yO3 (y=0 and 0.2) and LaCo1−xFexO3 (x=0.6 and 1) perovskites were used as catalysts for the total oxidation of methane in the temperature range 300–800°C and characterized by in situ electrical conductivity measurements. Their electrical conductivity was studied as a function of temperature and oxygen partial pressure and temporal responses during sequential exposures to air, methane–air mixture (reaction mixture) and pure methane in conditions similar to those of catalysis were analyzed. The intrinsic activity of the perovskites at 550°C followed the order: LaFeO3<LaCo0.8O3<LaCo0.4Fe0.6O3<LaCoO3. LaCo1−yO3 perovskites were in a metallic conductivity state in the reaction temperature range while LaCo1−xFexO3 perovskites appeared to be p-type semiconductors both under air and under reaction mixture with positive holes as the main charge carriers. A transition from semiconducting state to the metallic state was observed for LaCo0.4Fe0.6O3 in the reaction temperature range. Their electrical conductivity at 550°C increased following the order: LaFeO3<LaCo0.8O3<LaCo0.4Fe0.6O3<LaCoO3. Correlations between their conducting behavior and redox properties, on one hand, and their catalytic behavior, on the other hand, have been evidenced. It was demonstrated that the reaction mechanism involves surface lattice O− species and can be assimilated to a Mars and van Krevelen mechanism for LaFeO3, while it involves only adsorbed oxygen species for LaCo1−yO3, being assimilated to a suprafacial mechanism. Both suprafacial and redox mechanisms take place on LaCo0.4Fe0.6O3 perovskite.

Catalytic oxidation of CO on LaMn1−xFexO3 perovskites solid solution

Catalytic CO oxidation was studied over LaMn1−xFexO3 perovskites prepared by co-precipitation and rotor-vapor citrate method. The catalysts were characterized by X-ray diffraction (XRD), Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). XRD analysis showed that LaMn1−xFexO3 samples prepared by rotor-vapor citrate method were single-phase perovskite-type oxides, but not prepared by co-precipitation. XPS results indicated that Mn element was mainly in Mn4+ state and Fe element was mainly in Fe3+ state on the surface of the catalysts. LaMn1−xFexO3 prepared by rotor-vapor citrate method exhibited higher activity for CO oxidation than which prepared by co-precipitation, and among the LaMn1−xFexO3 series prepared by rotor-vapor citrate method LaMn0.4Fe0.6O3 showed highest activity due to highest BET surface area. The lowest apparent activation energy (Ea) towards the oxidation of CO over LaMn0.4Fe0.6O3 dues to the highest dispersion of Mn4+-O-Fe3+ species on the surface of LaMn0.4Fe0.6O3.

Surface reconstructions of LaCo1−xFexO3 at high temperature during N2O decomposition in realistic exhaust gas composition: Impact on the catalytic properties

This study is dedicated to the development of thermally stable LaCo1−xFexO3 perovskite based catalysts for the decomposition of N2O at medium and high temperature, in representative compositions of exhaust gas from ammonia burner in nitric acid plants. In these specific conditions, N2O coexists with a large amount of NOx. Up to now, lab-scale investigations were usually restricted to low amount of NOx and water while both can originate drastic changes in the catalytic performances related to activation/deactivation phenomena. The impact of NO concentration in terms of activity and stability has been discussed at moderate temperature and correlated to extensive bulk and surface characterization from different techniques (X-ray diffraction, Raman spectroscopy, XPS, Mössbauer spectroscopy, Tof-SIMS). It was found that the catalytic performances in terms of activity and stability depend on the relative concentration of Co with the best performances recorded on LaCo0.2Fe0.8O3.

Influence of Fe in giant magnetoresistance ratio and magnetic properties of La0.7Ca0.3Mn1−xFexO3 perovskite type compounds

In this work polycrystalline perovskite with composition La0.7Ca0.3Fe0.05Mn0.95O3 has been produced by standard ceramic method. A considerable change occurs in magnetic, transport and magnetoresistance properties with respect to the classic composition without Fe. The introduction of a different metal, Fe, in Mn-O layer causes a decrease of about 50 K in the value of TC. In the same way, a decrease of about 10–15% in the average magnetic moment measured at 1 T is also observed. On the other hand, the introduction of Fe does not cause any appreciable change in the value of the lattice parameter. This new compound presents 60% of giant magnetoresistance ratio at 200 K. Magnetization vs. temperature measurements in zero field cooling and field cooling show clear differences at applied fields below 80 kA m−1. Such a behavior, also observed in La0.7Ca0.3MnO3 sample, is not recognizable as a simple ferromagnetic one. A qualitative discussion of the effect in the magnetic and transport properties of these compounds, caused by the substitution of some Mn atoms by Fe is given.